Repeatability and interobserver error of digit ratio (2D:4D) measurements made by experts

Abstract

The second-to-fourth digit ratio (2D:4D) is sexually differentiated (lower in men than in women), a likely biomarker for organizational (permanent) effects of prenatal testosterone, and a correlate of many sex-dependent, hormonally influenced traits and phenotypes. The extent of 2D:4D measurement repeatability across different research groups is unknown. This study assessed the repeatability and interobserver error of 2D:4D measurements made by 17 experts (researchers who have contributed to the 2D:4D literature). Results indicate that 2D:4D, because it is a ratio variable, is notably less precisely measurable than finger length. Absolute-agreement intraclass correlation coefficients for these traits are about 0.75 vs. about 0.95, respectively. Associations of 2D:4D with target traits were usually of small size; measurement unreliability attenuates effects. This may explain some null findings and replication failures encountered in 2D:4D research. However, agreement levels are still sufficient to permit conclusions about findings from different research groups. One implication of this is that the marked geographical and population differences in typical 2D:4D levels, as reported in the literature, are veridical, and not due to divergent measurement habits across research groups. Some practical recommendations for 2D:4D measurement are offered.

Digit ratios (2D:4D) determined by computer‐assisted analysis are more reliable than those using physical measurements, photocopies, and printed scans

“…However, we are unable to locate any study that supported similar levels of interobserver agreement when determining 2D:4D from photocopied and printed hand scans. Although the level of agreement we reported for 2D:4D derived from printed scans was similar to findings made by Voracek et al (2007), the reliability coefficients were lower than those observed for photocopies and computerassisted image analysis. In our current study, we believe printed scanned images yielded less reliable 2D:4D because image quality was significantly lost at the time of printing-this occurred despite images being printed at the highest quality available to us (i.e., 600 dpi).…”

“…While each of the commonly used methods of acquiring 2D:4D is generally validated within studies to show they yield highly repeatable measurements among a single observer, it must be acknowledged that we know very little about the reliability of 2D:4D measurements made by multiple observers using a single technique (Voracek et al, 2007) or the reliability of measurements made by the same observers using multiple techniques (Kemper and Schwerdtfeger, in press;Manning et al, 2005 , 2005). These findings may be interpreted to mean that 2D:4D are technique dependent and this has serious implications for a field that necessitates a highly precise tool to identify relatively small differences in 2D:4D among study populations (Kemper and Schwerdtfeger, in press).…”

“…Printed scanned images and photocopies have the benefit of providing permanent records and shorter sampling times (Caswell and Manning, 2007;Manning et al, 2005); however, lower 2D:4D have been observed with photocopies and the equipment is not generally portable (Burriss et al, 2007;Manning et al, 2005). Digital image technology for the acquisition and measurements of finger lengths may provide a superior alternative to these more traditional techniques because: (1) large permanent databases can be generated requiring little physical space; (2) computer-assisted image enhancement can improve clarity of finger borders and creases which may improve measurement reliability and (3) digital technology may also be considered a more environmentally conscious alternative.While each of the commonly used methods of acquiring 2D:4D is generally validated within studies to show they yield highly repeatable measurements among a single observer, it must be acknowledged that we know very little about the reliability of 2D:4D measurements made by multiple observers using a single technique (Voracek et al, 2007) or the reliability of measurements made by the same observers using multiple techniques (Kemper and Schwerdtfeger, in press;Manning et al, 2005 , 2005). These findings may be interpreted to mean that 2D:4D are technique dependent and this has serious implications for a field that necessitates a highly precise tool to identify relatively small differences in 2D:4D among study populations (Kemper and Schwerdtfeger, in press).…”

“…Moreover, improving image quality in digital scans did not involve an excessive and unnecessary use of paper and toner, providing an environmentally sensitive alternative. The ability to scan hands into a computer also raises the issue of exciting future applications of image analysis including, semiautomated and automated determination of finger length measurements.We elected to print digital images and measure finger lengths using Vernier calipers because others have used this technique assuming this approach to be equivalent to that of photocopies (Voracek et al, 2007;Williams et al, 2003). However, we are unable to locate any study that supported similar levels of interobserver agreement when determining 2D:4D from photocopied and printed hand scans.…”

Females Have Larger Ratio of Second‐to‐Fourth Digits Than Males in Four Species of Salamandridae, Caudata

“…Measurements of digit ratio from photographs or scans are the most accurate methods (Manning, 2002;Voracek et al, 2007;Kemper and Schwerdtfeger, 2009;Ruuskanen et al, 2011) are difficult to use in the field (Manning, 2002;Navarro et al, 2007;Dreiss et al, 2008). On the front fore and hind limbs of newts there is no visible basal crease; thus, we had to create a standardized method of digits measurement.…”